/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */

// 迭代 时间 O(n) 空间 O(1)
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* head = new ListNode();
        ListNode* pos = head;
        while (l1 != nullptr && l2 != nullptr) {
            if (l1->val < l2->val) {
                pos->next = l1;
                l1 = l1->next;
            }
            else {
                pos->next = l2;
                l2 = l2->next;
            }
            pos = pos->next;
        }
        pos->next = l1 == nullptr ? l2 : l1;
        return head->next;
    }
};